1. Field of the Invention
The present invention relates to an ultrasonic motor for use as, for example, an image vibration correction unit of a digital camera or an actuator of an AF lens or the like.
2. Description of the Related Art
Generally, when a voltage is applied to a piezoelectric device of the ultrasonic motor, longitudinal vibration and flexural vibration are induced, thereby producing an elliptical vibration (oscillation). The ultrasonic motor transmits this elliptical vibration to a driven body via a driver so as to drive the driven body by friction.
A vibrational component using such a piezoelectric device has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 8-18379. According to Jpn. Pat. Appln. KOKAI Publication No. 8-18379, the piezoelectric device is formed as a piezoelectric vibrating body so that the piezoelectric vibrating body is sandwiched such that it can be vibrated by a holding frame. The piezoelectric vibrating body is sandwiched by a pair of cases and sealed such that it can be made to vibrate, thereby being prevented from being damaged by external pressure.
It has been demanded that the motor output of an ultrasonic motor having such a piezoelectric device be raised. To raise the motor output, it is necessary to increase a vibration induced by the piezoelectric device by, for example, increasing electric power applied to the piezoelectric device.
The structure disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-18379 is capable of preventing the piezoelectric device from being cracked by an external force with the holding frame. However, in such a structure, internal stress is concentrated by the vibration of the piezoelectric device. Thus, this structure cannot prevent the piezoelectric device from being cracked by the concentration of the internal stress. That is, when the motor output is increased so that the vibration of the piezoelectric device is intensified, the vibrational velocity resulting in cracks or destruction is raised, thereby inducing the cracking or destruction of the piezoelectric device due to the concentration of the internal stress, which is a problem inherent in this structure.